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The fatigue performance of Kaowool fiber reinforced 339 aluminum composites at 300°C is limited by spherical thin walled hollow Kaowool shot particles. These act as crack initiation sites particularly when located at the surface. This problem does not occur for thick walled particles or particles filled with the aluminum matrix. The effect of wall thickness (t) is evaluated from finite element analysis of both 2D and 3D models, with and without plasticity. Both models predict that hollow thin walled particles act as defects, while thick walled particles act as reinforcements, this transition being defined by a critical wall thickness (tc). The 3D model is preferred in that it predicts more accurate and smaller values of tc. Specifically, the 3D elastic/plastic model predicts that the largest stress concentration occurs for a fractional surface particle and that in this condition tc = 0.18a, where a is the particle radius. This value agrees with our experimental observation that particles with t > 0.2a do not initiate failures.